Separation of powers is a theory mostly considered by montesqueau (sp?) not a convention
Examples of important conventions...
* ministerial responsibility, both individual and collective * Queen chooses the PM but by convention chooses the leader of the party with the majority in the House of Commons * Royal Assent to legislation (though this has waned in importance) * Parliament dissolves every 7 years (since the Septennial Act) but by convention, the PM asks the Queen to dissolve it